Calico Cats

By definition calico cats are tri-colored with white, black, and orange.  Here is a picture of Sally Cowan's little girl, Twinkle doing her best "chomp chomp, Go Gators!" cheer. (Photo credit to Sally Cowan, http://www.felinefoto.com/)

Twinkle is black and orange and white and a girl...the perfect combination for a calico cat.  Why are they all girls (almost)?  Can there be boy calicoes?  

First let me answer the second question, there can be but it is an unusual set of circumstances and if you are ever in a betting situation, put your money on the cat being female!  To have a male calico would require the fusion of two fertile eggs (zygotes), not identical twins,  and the development of only one viable kitten.  Could?  Yes, but VERY unlikely.

Now the interesting part.  The gene for white fur is carried on some chromosome other than the X or Y.  The Y chromosome, just as in humans, carries only a few genes and these are for boy things--in humans, for example, the gene for big tires, hogging the remote, and loud trucks are all located on the Y chromosome (kidding).

The X chromosome is not really just the female chromosome.  It carries genes for certain female traits, but also genes for other traits just like any other chromosome.  The gene for pattern-baldness, for example, is carried on the X chromosome.  Well in cats the gene for black fur and for orange fur are located in exactly the same place on the X chromosome.  (That means they are alleles of the same gene.)  Obviously one X chromosome can only have either a black or an orange gene (allele).

Here is the tricky part.  In all females that have the two X system of sex determination, one of the X chromosomes is inactivated in each cell.  Yes, it can be a different X chromosome that is inactivated in any given cell.  (The inactive X chromosome is called a Barr Body.)  So you have a cat that is heterozygous for black and orange--one X chromosome has a black gene, one has an orange gene.  In the skin cells where the "orange" X is inactivated, the fur is black.  In skin cells where the "black" X is inactivated, the fur is orange.  Throughout the cat there are patches of orange and black and filled in with white here and there!  And, with two X chromosomes it is female.  VIOLA, female calico.

Had Twinkle been a boy, his name would probably not have been Twinkle and he would have had only one
X chromosome.  What color would he have been?  Right!  He would have been black and white or orange and white.

Isn't that the coolest thing you ever heard of?

Watch for another post soon about why Siamese, Persians, and Himalayans often have color points on ears, nose, feet and tail.

~klock